Mycobacterium tuberculosis is one of the most successful human pathogens known. To achieve such success, M.tuberculosis must avoid destruction by the immune system. Nevertheless, its mechanisms of evading the immune system are not well understood. The PI tested the hypothesis that M.tuberculosis evades immune effector function by inhibiting the action of interferon gamma, a crucial mediator of innate immunity. The investigators found that M.tuberculosis infection of human macrophages blocks transcriptional activation of interferon gamma responsive genes, and that it does so by disrupting the interaction of the transcription factor, STAT 1, with the transcriptional coactivators CBP and p300. By blocking interferon gamma transcriptional responses, M.tuberculosis can inhibit activation of macrophages and can survive despite development of a cellular immune response. In this application experiments are proposed to identify the mechanism by which M.tuberculosis disrupts the STAT1-CBP interaction, and to determine whether overcoming those mechanisms allows human macrophages to respond to interferon gamma by killing M.tuberculosis. The effects of live M.tuberculosis can be replicated by gamma-irradiated bacteria, as well as a by a crude cell wall fraction, but not by LAM. We propose to identify the specific components of the M.tuberculosis cell wall that initiates inhibition of interferon gamma responses, and to characterize macrophage responses to that component. The investigators have also discovered that, in response to M.tuberculosis, macrophages synthesize and release a soluble protein (termed SINGR) that causes inhibition of interferon gamma responses in naive, uninfected cells. SINGR activity is not attributable to known cytokines, such as IL-4, Il-6, IL-10, or TGF-b. The investigators propose to identify SINGR by purification or expression cloning, prepare antibodies that neutralize its activity, and determine whether SINGR is an obligate intermediate molecule in the inhibition of interferon gamma responses by M.tuberculsois. It is hoped that the results of these studies will provide valuable insight into the interaction of M.tuberculosis with the human immune system, particularly the ability of this pathogen to persist in the face of a seemingly appropriate immune response.